Skip to content
Fencer Intel
medium 5.3

CVE-2017-7820 json

The "instanceof" operator can bypass the Xray wrapper mechanism.

published 2018-06-11 8 y ago modified 2026-06-17 4 mo ago also EUVD-2017-16795

cvss
5.3
version 3.0, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.2%
EPSS 1.2% chance of exploitation in 30 days, percentile 67%
public exploits
0
none known

Summary

The "instanceof" operator can bypass the Xray wrapper mechanism. When called on web content from the browser itself or an extension the web content can provide its own result for that operator, possibly tricking the browser or extension into mishandling the element. This vulnerability affects Firefox < 56.

Patch / fix

  • Mozilla Firefox 56
  • firefox Ubuntu 14.04 LTS 56.0+build6-0ubuntu0.14.04.1
  • firefox Ubuntu 16.04 LTS 56.0+build6-0ubuntu0.16.04.1
  • firefox Ubuntu 18.04 LTS 56.0+build6-0ubuntu1

ubuntu advisories UBUNTU-CVE-2017-7820

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • firefox-esr openSUSE Tumbleweed 128.5.1-1.1
  • MozillaFirefox openSUSE Tumbleweed 92.0-1.2

opensuse advisories openSUSE-SU-2024:14572-1openSUSE-SU-2024:10600-1

Exploit prediction

1.16% chance of exploitation in the next 30 days, higher than 67% of all scored CVEs (FIRST EPSS, 2026-11-09).

Exploits and checks

  • bugzilla.mozilla.org https://bugzilla.mozilla.org/show_bug.cgi?id=1378207 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
NVD 3.0 5.3 CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
NVD 2.0 5.0 AV:N/AC:L/Au:N/C:N/I:P/A:N

Affected products

  • Mozilla Firefox

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.